1. Field of the Invention
Embodiments of the present invention relate generally to internal combustion engines and, specifically to internal combustion engines with mechanisms for varying the compression ratio.
2. Background of Related Art
In a reciprocating internal combustion engine, the compression ratio of an engine is defined as the ratio between the free volume of the cylinder when the piston is at bottom-dead-center (BDC) to the free volume when the piston is at top-dead-center (TDC). All other things being equal, engines tend to be more efficient and produce more power when run at higher compression ratios because this results in higher thermal efficiency. Diesel engines, for example, run at very high compression ratios (18:1 and higher) resulting in compression ignition (i.e., spark plugs or other ignition sources are not required to light the fuel). The higher compression ratio of diesel engines, along with the slightly higher heat content of diesel fuel, results in an engine that provides significantly better fuel mileage than a comparable gasoline engine.
In a gasoline engine, however, increasing the compression ratio is limited by pre-ignition and/or “knocking” In other words, if the compression ratio is high enough then, like a diesel, the compression of the fuel causes it to ignite (or, “pre-ignite”) before the spark plug fires. This can result in damage to the engine because cylinder temperatures and pressures spike as the fuel/air mixture explodes on multiple fronts, rather than burning uniformly. The maximum acceptable compression ratio in an engine is limited by a number of factors including, but not limited to, combustion chamber and piston design, cylinder and piston cooling, engine loading, and air temperature and humidity. The maximum compression ratio used in production engines is generally relatively conservative (on the order of 10.5:1 for cars and 12.5:1 for motorcycles) to account for, for example, the wide variety of operating conditions and fuel quality.
Due to difficulties associated with reliably moving components in an operating internal combustion engine, however, all currently mass produced engines operate with a fixed compression ratio. As a result, the stock compression ratio tends to be a compromise between a high-compression ratio, which is more efficient—but can result in the aforementioned knocking—and a low compression ratio engine—which is more forgiving of, for example, poor quality fuels, high loads, and/or high temperatures.
The ability to change compression ratio during operation can improve fuel efficiency 35-40% and more. When under light load, for example, such as when the vehicle is cruising down the highway, the compression ratio can be increased significantly to increase fuel mileage. When the engine is under a heavy load, ambient air temperature is very high, or fuel quality is low, on the other hand, the compression ratio can be reduced to prevent knocking.
A number of designs exist that have attempted to vary the compression ratio of an internal combustion engine in use. Patents have been filed on variable compression ratio (VCRE) engines for over 110 years. A few of the proposed VCRE engines are based on the concept of raising and lowering the cylinder block/head assembly portion of an engine relative to the crankcase. In this configuration, the distance between the piston at top-dead-center (TDC) and the cylinder head can be varied, thus varying the compression ratio of the engine.
Prior inventions based on raising and lowering the cylinder block/head assembly relative to the crankcase have not been practical for use in moving vehicles, however. Prior inventions allowed the cylinder block/head assembly to move in substantially all directions (i.e., as opposed to limiting movement to the Y axis, or perpendicular to the crankshaft), resulting in severe side loading and premature component failure. Other previous mechanisms have separated the cylinder sleeve from the crankcase, used heavy control mechanisms, or have prevented the location of engine mounts above the center of gravity of the engine leading to stability issues. Still other inventions have incorporated a continuous and closed crankcase housing extending above a traditional crankcase and enclosing the cylinder block, for example, which was heavy and created challenges in eliminating the heat generated by the engine. Finally, prior art solutions have eliminated the critical role cylinder head bolts play in transferring forces between the cylinder head, cylinder block, and crankcase.
What is needed, therefore, is a system for varying the compression ratio of an internal combustion engine without unnecessarily increasing the weight or complexity of the engine. The system should enable the block and head assembly to move vertically with respect to the crankcase, while substantially constraining the engine in all other directions. The system should use conventional manufacturing techniques to provide easily manufacturable, reliable engines with, among other things, improved power-to-weight ratios and fuel consumption. It is to such a system that embodiments of the present invention are primarily directed.